Homo- or Heterosynthon? A Crystallographic Study on a Series of New Cocrystals Derived from Pyrazinecarboxamide and Various Carboxylic Acids Equipped with Additional Hydrogen Bonding Sites

Four new cocrystals of a well studied active pharmaceutical ingredient (API), namely, pyrazinecarboxamide ( PZA ), with various monocarboxylic acids equipped with additional hydrogen bonding sites such as vanillic acid ( VA ), gallic acid ( GA ), 1-hydroxy-2-naphthoic acid ( 1HNA ), and indole-2-carboxylic acid ( I2CA ) have been successfully prepared and characterized by FT-IR, 1 H NMR, differential scanning calorimetry (DSC), and single crystal and powder X-ray diffraction (SXRD and PXRD, respectively) techniques. In the majority of the cases, preferential occurrence amide–amide and acid–acid homosynthons has been observed. Since the heterosynthon is energetically preferred to homosynthon, such unusual occurrence of homosynthon in these cocrystals is intriguing.